Sink marks and voids are usually treated as two different defects. They’re really two outcomes of one event: a thick section of plastic shrinking as it cools. The outside skin of a molded part freezes first; the material in the core is still hot and shrinks as it solidifies, and it has to pull that volume from somewhere. If the skin is still soft, it gets pulled inward and you see a sink mark on the surface. If the skin has already frozen solid, it can’t move, so the shrink opens up an internal vacuum bubble — a void — sealed inside the part.
That shared root cause is the most useful thing to understand about both. Sink and voids are a too much thick material, not enough packing problem, and the most reliable fixes live in part design, not in cranking pressure at the press. A processor can reduce sink with pack pressure, but if the wall is simply too thick, they’re fighting the part’s geometry every shot for the life of the job.
Sink versus void: how to tell which you have
| Sink mark | Void | |
|---|---|---|
| Where | On the surface, usually opposite a thick feature (a rib, boss, or thick wall) | Inside the part, hidden under a frozen skin |
| What you see | A visible depression or dimple | Nothing on the surface — found by sectioning, backlighting, or on translucent parts |
| What “won” | Skin still soft, so it pulled in | Skin froze first, so the core tore open internally |
| Tends toward | Hotter mold, thinner skin, slower freeze | Colder mold, thicker section, fast skin freeze |
Both reduce part quality. Sink is usually a cosmetic reject; a void can be both cosmetic (on clear parts) and structural, because it’s a missing chunk of material right where the section was supposed to be solid.
The cause tree
Because sink and voids share a cause, they share a cause tree. The biggest levers are in design; process can compensate within limits.
| Cause area | What’s happening | Fix |
|---|---|---|
| Design — thick wall | A wall or section too thick to pack out and cool evenly | Thin and even the wall; core out heavy sections |
| Design — ribs/bosses | A rib or boss as thick as the wall it joins shrinks at the junction | Keep ribs around half to two-thirds of the adjoining wall |
| Design — abrupt transitions | Sharp thick-to-thin changes concentrate shrink and stress | Blend transitions gradually rather than stepping |
| Process — pack | Not enough pack pressure or time to feed the shrinking core | Raise pack pressure; extend pack time to gate seal |
| Mold — gate | Gate too small or far from the thick section; it freezes before packing finishes | Gate into the thickest section; size the gate for adequate pack time |
| Material | High-shrink resin amplifies the effect | Account for shrink; consider a filled or lower-shrink grade |
Why thinning the wall beats chasing pressure
The reflex fix for sink is to raise pack pressure, and within limits that works — more pressure feeds more material into the shrinking core before it freezes. But there’s a ceiling. Once the gate freezes (gate seal), no amount of machine pressure reaches the cavity, and a thick section keeps shrinking after that point no matter what the press is doing. If the wall is genuinely too thick, you’re packing against physics.
The design rules that actually solve it:
- Keep walls thin and uniform. Thick walls are the root of most sink and voids. Uniform walls cool evenly and shrink evenly. As an illustration of the principle, walls much above roughly 2–3 mm start to invite sink and thickness shadows on cosmetic surfaces, and uniformity matters as much as absolute thickness — defer exact limits to the resin and the part.
- Size ribs and bosses to the wall. A rib that’s as thick as the wall it joins creates a thick spot at the junction that sinks on the show surface. Keeping rib thickness to roughly half to two-thirds of the adjoining wall keeps the junction from becoming a heavy section.
- Core out heavy sections. A solid thick boss or block should be cored to a uniform wall around a hollow, rather than left as a mass that will shrink internally.
- Blend transitions. Step a wall abruptly from thick to thin and you concentrate shrink and stress right at the step; a gradual transition spreads it.
Design fixes are permanent and free per part once the tool is right. Pressure is a per-shot fight with diminishing returns.
Process and gate levers that genuinely help
When the geometry is reasonable and you still see sink or voids, the process and gate side has real levers:
- Pack pressure and time. Pack to gate seal — long enough to feed the core until the gate freezes, no longer. Too little pack starves the section; the right amount feeds it before freeze.
- Gate into the thick section. If the gate feeds a thin area and the melt has to travel to the thick section, the path may freeze before the thick part is packed. Gating into or near the heaviest section lets pack pressure reach where the shrink is.
- Gate size and freeze timing. A gate too small freezes early and cuts off packing prematurely. Confirm the gate stays open long enough via a gate-seal study.
- Mold and melt temperature. These shift the balance between sink and void and affect how the skin freezes; tune them, but they won’t rescue a section that’s simply too thick.
A practical sequence
- Identify sink or void, and locate it relative to thick features (ribs, bosses, heavy walls).
- Check the geometry first. Is there a thick section, a heavy rib/boss junction, or an abrupt transition driving it? If so, that’s the real fix.
- For a reasonable geometry, confirm pack pressure and time to gate seal, and that the gate feeds the thick section and stays open long enough.
- Tune mold/melt temperature to shift the skin-freeze balance.
- If geometry is the cause, flag it as a design correction (thin the wall, size the rib, core the section) rather than living with a per-shot pressure fight.
FAQs
What’s the difference between a sink mark and a void?
They come from the same cause — a thick section shrinking as its core cools — but differ in whether the skin holds. If the surface skin is still soft when the core shrinks, it gets pulled inward and you see a sink mark on the surface. If the skin has already frozen solid, it can’t move, so the shrinking core opens an internal vacuum bubble (a void) hidden inside the part. Sink is on the surface; a void is internal and often invisible unless you section the part or it’s translucent.
Why doesn’t more pack pressure fix my sink marks?
Because pack pressure only works until the gate freezes. Once the gate seals, no machine pressure reaches the cavity, and a section that’s too thick keeps shrinking after that point regardless of what the press does. Raising pack helps within limits, but if the underlying cause is a wall or boss that’s simply too thick, you’re fighting the geometry every shot. The durable fix is to thin and even the wall, size ribs to the wall, or core out the heavy section — design changes that pressure can’t substitute for.
How thick is too thick for a wall?
It depends on the resin and the part, so treat any single number as illustrative rather than a rule — but uniformity matters as much as absolute thickness. Walls much above roughly 2–3 mm start to invite sink and thickness shadows on cosmetic surfaces, and abrupt changes from thick to thin concentrate shrink and stress. The practical aim is a thin, uniform wall with gradual transitions, ribs kept to about half to two-thirds of the wall they join, and heavy solid sections cored out. Defer exact limits to the material datasheet and a flow analysis.
Why do sink marks show up opposite ribs and bosses?
Because a rib or boss adds material at its junction with the wall, creating a locally thick spot. That thick junction cools and shrinks more than the surrounding thin wall, and the extra shrink pulls the opposite (show) surface inward — a sink mark that mirrors the rib on the back. Keeping rib and boss thickness to roughly half to two-thirds of the adjoining wall keeps the junction from becoming a heavy section, which is why that ratio is a standard design guideline for avoiding read-through sink.